This invention relates to the calendering of webs formed of paper or similar substances. Paper webs used, for example, in the manufacture of magazines, newspapers and the like, must be calendered for the purpose of polishing the surface so that it will accept ink properly and be easily readable. Calendering is the act of polishing the web by passing it through one or more nips of a calender machine. The nips are formed by pairs of opposed rollers.
There are several types of calendering operations. One type, known as supercalendering, consists of passing a web sequentially through a series of nips formed by pairs of iron and filled rolls. The term supercalendering is used because there may be eight or more such nips through which the web passes. Supercalendering can impart useful properties to the web including improved gloss, density, smoothness and the like.
Another type of calendering is the single or dual pass machine calender in which relatively few nips formed by pairs of iron rolls are employed as, for example, one or two nips. Such calendering is satisfactory for some grades of paper but is suited for manufacturing paper with high gloss and smoothness for quality printing.
If possible, of course, it would be preferred to put a web through as few nips as possible to obtain the desired characteristics necessary for quality printing.
A web passing through multiple nips becomes increasingly less able to tolerate additional processing without breaking, tearing, cracking, etc. Accordingly, it would be highly desirable to produce a quality paper without the need for supercalendering a web. In short, it would be desirable to produce quality paper from a calendering operation involving only a few nips, say two or less.
It is known in the prior art to temperature gradient calender a web for the purpose of obtaining improved gloss, smoothness, strength and ink transfer capabilities. Such knowledge is described in an article authored by R. H. Crotogino, published in the Tappi Journal dated October, 1982, at pages 97 through 101. The Crotogino article describes a calendering operation in which a web is passed through a pair of nips formed by heated iron rolls. The rolls are heated to a temperature of approximately 210.degree. C. (410.degree. F.). By temperature gradient calendering it is meant that there is a significant difference between the temperatures to which the web surface and the interior of the web are subjected. Thus, when a relatively cold web contacts the very hot iron rolls, a significant temperature differential exists between the surface of the web and the interior portion. When the web is compressed by the nips of the heated rolls the hot outer web surface is deformed more than the interior resulting in a smoother, glossier web which is higher in strength and has better ink transfer capability than webs which are machine calendered at moderate temperature. Indeed, Crotogino suggests that nip temperature gradient calendering can approach the quality obtained by supercalenders with respect to smoothness and gloss.
The Crotogino device obtains a constant caliper of the web but not constant density. This is due primarily to the rigidity of the heated iron rolls. This lack of density control results in mottling of the web. By mottling it is meant that deviations inherent in a web (high and low spots) are not treated equally by the nip. Thus, high spots are made glossier and smoother than the low spots which are not subjected to the same temperature and pressure.
It is desirable to provide a method and apparatus which has the benefits of temperature gradient calendering--few nips--but can produce high quality webs without mottling.
It is accordingly an object of the present invention to provide an improved method and apparatus for temperature gradient calendering which can obtain the advantages of temperature gradient calendering, namely few nips, while at the same time producing a high quality web of constant density and little or no mottling.
It is a further object of the invention to provide a two nip temperature gradient calendering method and apparatus utilizing a heated iron roll in conjunction with an unheated compliant roll whereby both sides of a web can be treated.
A further object of the invention is to provide a temperature gradient calendering method and apparatus employing a crown control compliant roll to maximize the quality of the polishing action.
Other objects and advantages of the invention will be apparent from the remaining portion of the description.